To add a few comments from my reading of books on the GT40's, originally in 1964 the Mk 1 GT40's were getting 700 lb (total front and rear) of lift near top speed. This was described as an aerostability problem causing a rotary motion of the rear end which increased with speed and was accentuated on a wet road, so that eventually the rear end lost adhesion. The Lola GT had suffered similar problems. Earlier wind tunnel work had "proved" that a spoiler would be superfluous, but practical know-how and test driving at the MIRA proving ground lead to the addition of a rear spoiler. The stability of the car was described as transformed. Then later in 1965, when the Mk2's (without spoilers) could exceed 200mph, they were horribly unstable at Le Mans at speeds approaching this. Spoilers and stabilising fins (and canards) were hastily improvised and added. Finally, the rear anti-roll bar was dispensed with (removal presumably increased the understeer) and handling became acceptable at Le Mans.
All a bit academic for road cars, but obviously an interesting subject for those forum members who race, or for those trying to replicate a particular car configuration.
Incidentally, original design estimates had suggested a maximum speed (Mk 1) around 210 mph. But in practice 76 rather than the estimated 30 bhp were absorbed in ducting to brakes, radiator, and engine, so that the true still-air maximum was 197 mph, according to one source.
Dalton